1. Field of the Invention
The invention relates to an electric power steering apparatus that transmits assist force from an electric motor to a rack shaft via a ball screw.
2. Description of Related Art
An electric power steering apparatus that uses a ball screw is described in Japanese Patent Application Publication No. 2005-145431 (JP-A-2005-145431).
An electric power steering apparatus 100 shown in FIG. 7 has a structure in which an assist force from an electric motor is transmitted to a rack shaft 110 via a driving pulley 102, a timing belt 103, a driven pulley 104 and a ball screw 105. Two opposite ends of the rack shaft 110 are linked to tire wheels via ball joints (not shown) so that the direction of the wheels can be changed by movement of the rack shaft 110 in the axial direction of the rack shaft 110. A steering wheel (not shown) is rotatably linked to rack teeth formed on the rack shaft 110, via a pinion shaft (not shown), so that rotation of the steering wheel is converted into movement of the rack shaft 110 in its axial direction.
In order to smoothly transmit the assist force from the electric motor 101 to the rack shaft 110, the timing belt 103 needs to be provided with an optimum belt tension. In the electric power steering apparatus 100, the driving pulley 102 is rotatably journaled to an adjustment collar 111, at a position on the adjustment collar 111 which is apart from the axial center of the adjustment collar 111. In the electric power steering apparatus 100, when the belt tension of the timing belt 103 is optimized by turning the adjustment collar 111, a locknut 112 is screwed to an external screw 111a of the adjustment collar 111. In the electric power steering apparatus 100, the adjustment collar 111 is fixed to a gear housing 113, with the angle of the adjustment collar 111 maintained.
The electric power steering apparatus 100 employs an eccentric adjustment mechanism that is made up of the adjustment collar 111, a rotating shaft 114 linked coaxially with a drive shaft 113 of the electric motor 101, a bearing 115 that rotatably supports the rotating shaft 114, and the locknut 112. Since in the electric power steering apparatus 100, the drive shaft 113 of the electric motor 101 and the rotating shaft 114 are linked together after the belt tension is adjusted, the belt tension adjustment can be easily accomplished; however, this method requires a relatively large number of component parts, resulting in a complicated structure, and therefore requires a large amount of time for the assembly.
If instead of using the foregoing eccentric adjustment mechanism, a driving pulley is attached to a drive shaft of an electric motor and then the electric motor is fixed to a gear housing while the belt tension is being adjusted, the number of component parts will be reduced, and the structure will be simplified. However, when the timing belt is to be wrapped around the driving pulley, it is necessary to incline the electric motor, which results in interference of the electric motor with the gear housing. Thus, as for this method, the efficiency of the assembling operation is low.